Over the last several decades, significant advancements have been made in personal computer (PC) microprocessor architecture and fabrication techniques, to improve performance and control costs. With each successive generation, an increasing number of functions have been integrated into a processor integrated circuit die. This increase in density has been enabled by shrinking the size of the constituent transistor elements, thereby allowing more transistors per unit area on the die. Performance improvements have been obtained by for example increasing the clock frequency at which certain functions of the processor operate. As a result, each successive generation of such processors is more powerful from a total performance standpoint, and has increasing overall power consumption.
The power consumption of a processor is proportional to a product of its power supply voltage and current. Successive generations of processors with increasing transistor density operating at higher speeds (e.g., higher dock frequency) demand a lower power supply voltage but higher power supply current. For example, in the early days of microprocessors offered by Intel Corp. of Santa Clara, Calif., transistor count per die was in the low 100,000s, processor clocks were running at around 100 MHz, supply voltages were at 5V DC, and supply current was no more than 10 A (depending on the activity level of the processor). With more recent Pentium class processors, transistor count per die is well above 1 million, clocks are in the GHz range, the power supply voltage needs to be dropped to about 1.2V DC or less, while current draw (at high activity levels) easily surpasses 100 A.
The supply voltage needs to be regulated to stay within a certain range, in the presence of operating temperature variations and as the processor transitions between different activity levels. However, the smaller supply voltages have resulted in tighter ranges being required, in the face of large current swings. This has led to many challenges in providing low cost power delivery for advanced processors.
To ensure reliable power delivery for its microprocessor families, Intel has set voltage regulator design guidelines. A voltage regulator (VR) is an electronic circuit that draws current a power source, to feed the processor and maintain a well-regulated power supply voltage for the processor. The VR maintains a setpoint voltage (e.g., at a so-called “Vcc” node of the processor), using a feedback control loop that repeatedly senses deviations from the setpoint, and corrects for them by increasing or decreasing the amount of current drawn from the source. For greater power conversion efficiency, switching-type regulators are used that draw current from the source using transistor or similar devices that turn on and off at high rates and stay on for relatively short pulses. The setpoint voltage is maintained by suitably controlling the pulse widths or duty cycle.
Conventional switching VRs for high performance processors have more than two phases or paths to the power source, through which current is drawn to maintain the supply voltage. The multiple phases help reduce the magnitude of voltage and current ripple, which enables the use of smaller phase inductors and power transistor switches, as well as smaller filtering and decoupling components at the output of the VR. In addition, a multi-phase switching VR can be made to respond quickly to deviations from the setpoint voltage (both voltage droops and voltage spikes), thereby helping maintain the smaller supply voltages within tighter tolerance bands.
Conventional multi-phase VRs have a fixed phase-firing order. The phases are all fired at the same frequency, but time shifted relative to each other. For example, a 100 kHz two-phase regulator will have both phases switching at 100 kHz, with the assertion edges of the two phases separated by five microseconds (one-half the switching period). This approach, however, is problematic when the load current to the VR (for example, the power supply current to a processor) is in essence synchronized to the switching frequency. For instance, a periodic load of five microseconds high current, five microseconds low current creates a serious current imbalance in the two-phase 100 kHz VR. If left unchecked, the current imbalance may quickly reach the per-phase current-limit and usually lead to some form of automatic shutdown of the VR which causes a catastrophic error during operation of the computer system.
There are conventional schemes to reduce the fatal current imbalance, where such schemes typically rely heavily on bulk capacitance at the output, to reduce the need for temporal alignment of current into the VR versus current out of the VR. In one case, a phase current is compared with an average current (that represents the average current amongst all of the phases), and the duty cycle is controlled on that basis to minimize the difference of current in each phase.